The present invention generally pertains to the collection of fine particles from a gas stream and is particularly directed to a fine particle collection method and system wherein the gas stream is moved through a shallow bed of solid granules.
Two prior art particle collection systems that are related to the present invention are perforated plate wet scrubbers and fluidized beds of solid collector granules.
The collection of fine particles from gas streams by means of perforated (sieve) plate wet scrubbers is well known. Gas flows generally upward through the perforations in the sieve plate and thence through a layer of agitated liquid (foam or froth) and up through another plate or out of the scrubber. Fine particle collection efficiency is known to occur by inertial impaction and interception and can be augmented by the use of other forces which tend to cause particle separation from the gas phase.
During the formation of bubbles on a sieve plate, contact between gas and liquid phases is due to the jets of gas emerging from the perforations and impacting on the liquid. Particles are thus deposited on the liquid surface by inertial impaction. Particle collection can be predicted from the following equation, which is based on experimental data: ##EQU1## WHERE F=foam density, fraction of volume that is liquid
D.sub.p =particle diameter, cm PA1 .rho..sub.p =particle density, g/cm.sup.3 PA1 C'=cunningham slip correction factor PA1 U.sub.h =gas velocity in the perforation, cm/s PA1 .mu..sub.G =gas viscosity, Poises PA1 D.sub.h =diameter of perforation, cm PA1 Pt.sub.d =penetration of particles of diameter d.sub.p, for collection during bubble formation
Perforated plate wet scrubbers have several disadvantages including the following. The scrubber liquid and solutions which are formed in the scrubber cause corrosion. Solids deposit and adhere to the plates due to mechanisms which act in the presence of water. A relatively high minimum liquid to-gas ratio is required to maintain the necessary foam height on the plate. Gas absorption occurs whether one wants it or not. This can be undesirable in applications such as the cleaning of power plant flue gas from the combustion of low sulfur coal, when particle collection is desired but the gas absorption is not. The operating temperature is limited by the vapor pressure of the scrubber liquid. Liquid waste treatment and/or disposal can be difficult and expensive. When liquid particles are being collected the particulate liquid will be diluted by the scrubber liquid.
Fluidized beds of solid collector granules, including spouted fluidized beds, have been used to collect fine particles from gas streams. These also have several disadvantages as follows: Prior art fluidized beds rely on particle collection throughout a deep bed with comparatively low gas velocities relative to the bed granules. High gas-phase pressure drop is required to maintain fluidization in a deep bed. Deep fluidized beds involve violent circulation of the bed granules and consequent attrition and therefore reentrainment of fine particles which have been collected. There is also some attrition of the bed granules themselves and the formation of fine particles. Deep fluidized beds involve a large inventory (holdup) of bed granules. This can have undesirable consequences in terms of cost, heat requirements, start-up characteristics, etc. Deep fluidized beds require larger equipment volume and larger static and dynamic loadings due to the mass of solids. Consequently, the equipment must be more massive and therefore more expensive.